Discussion

Some of the assumptions on which the modelling is based are unrealistic but necessary to compare the effect of control by killing versus reproductive inhibition. In fact, invalidity of some of the assumptions is likely to affect the modelling of control by conventional means more than sterilisation. Thus, the impact of immigration is likely to be felt immediately following reduction of numbers by conventional control, but not following any sterilisation which leaves animals to compete for resources. Furthermore, it is unlikely that a kill of 70% could be obtained for three successive years (because of the likely reduced acceptance of baits by a decreased population), but it is more reasonable, provided members of a population accept baits equally, to expect baits carrying a permanent sterilant to be accepted by 70% of the population for three successive years. (Bait shyness may affect acceptance of repeated applications of temporary chemosterilants but not permanent sterilants).

The pattern of results is the same for possum populations with other maximum rates of increase (and is the same as shown by Knipling and McGuire (1972) for rats). The slower the rate of increase of a possum population the more effective a control operation will be (i.e. the longer it will take the population to recover).

A maximum rate of increase of 1.4 is likely to be in excess of what a possum population would reach in today's modified New Zealand habitats page 231 (although it is quite possible that in the early days of colonisation such rates were reached). Starting with a population of 30 possums with a maximum rate of increase of 1.4, a colonising population is estimated (by the model) to take 23 years to reach 90% of its maximum density for the habitat. In fact populations typically over-shoot the maximum density, modify the habitat, and crash from this peak to a lower level. Pracy (1977) has noted that no population has yet stabilised in New Zealand.

Most of the early large-scale poison operations were aimed at peak or near peak possum populations (rather than stable asymptotic ones). If these populations would have naturally crashed to 50% of their peak level, then a kill of 50% only achieves what would have happened naturally. Such an example appears to have occurred in the Kokatahi (Boersma 1974). This may partially explain why no monitored populations are known to have increased significantly following large-scale poisoning operations (see also Bamford 1973).

The advantage of killing is that it gives immediate results. However, with the current use of poisons there are also problems with poisoning non-target species. Temporary sterilisation has no immediate or marked effect on a population, but is the safest for non-target species. However, it is unlikely that a temporary, moderate reduction in the numbers of a pest species will ever be required. Permanent sterilisation of one sex has no theoretical advantage over killing both sexes (Knipling and McGuire 1972). Permanent sterilisation of both sexes (whether by the same compound for both sexes or different compounds for each sex) is the most effective way of controlling population numbers. It is equally dangerous to target and non-target species. Thus, if permanent sterilants are to be used they will need to be highly specific or in a highly specific carrier-bait, and this is probably the main reason why they are not more widely used at present (see also Jackson 1972). Even then, a permanent sterilant will not give immediate results, and may need to be combined with conventional killing.